Polishing apparatus and polishing method

ABSTRACT

A polishing apparatus according to an embodiment includes a polishing table; a polishing pad provided on the polishing table; a first rotating mechanism configured to rotate the polishing table on a first rotation axis; a substrate holding unit configured to hold a substrate and press the substrate against the polishing pad; a second rotating mechanism configured to rotate the substrate holding unit on a second rotation axis; and a tilting mechanism configured to change an angle between the first rotation axis and the second rotation axis. In the apparatus, the circumferential edge of the substrate is always kept inside the circumferential edge of the polishing pad during polishing of the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-052786, filed on Mar. 20, 2019, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a polishing apparatus and a polishing method.

BACKGROUND

Chemical mechanical polishing (CMP) is a method for polishing the face of a substrate. In CMP, a polishing agent is supplied onto a rotating turntable, and then a substrate being rotated is pressed against a polishing pad provided on the turntable in order to polish the face of the substrate. The face of the substrate is thus chemically and mechanically polished and planarized.

In CMP, the polishing rate sometimes differs between the central area and the circumferential area of a substrate, which may cause nonuniform polishing of the face of the substrate. Under the circumstances, a method for adjusting the polishing rate across the face of a substrate has been required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of a polishing apparatus according to a first embodiment;

FIG. 2 is a schematic diagram of a polishing apparatus according to a second embodiment; and

FIG. 3 is a schematic diagram of a polishing apparatus according to a third embodiment.

DETAILED DESCRIPTION

A polishing apparatus according to an embodiment includes a polishing table; a polishing pad provided on the polishing table; a first rotating mechanism configured to rotate the polishing table on a first rotation axis; a substrate holding unit configured to hold a substrate and press the substrate against the polishing pad; a second rotating mechanism configured to rotate the substrate holding unit on a second rotation axis; and a tilting mechanism configured to change the angle between the first rotation axis and the second rotation axis. In the apparatus, the circumferential edge of the substrate is always kept inside the circumferential edge of the polishing pad during polishing of the substrate.

With reference to the drawings, embodiments of the disclosure will now be described. In the following description, the same or similar components are provided with the same reference numerals and repeated description of the same or similar components may be omitted.

With reference to the drawings, polishing apparatuses and polishing methods according to embodiments will now be described.

First Embodiment

A polishing apparatus according to a first embodiment includes a polishing table; a polishing pad provided on the polishing table; a first rotating mechanism configured to rotate the polishing table on a first rotation axis; a substrate holding unit configured to hold a substrate and press the substrate against the polishing pad; a second rotating mechanism configured to rotate the substrate holding unit on a second rotation axis; and a tilting mechanism configured to change the angle between the first rotation axis and the second rotation axis. In the apparatus, the circumferential edge of the substrate is always kept inside the circumferential edge of the polishing pad during polishing of the substrate.

A polishing method according to a first embodiment includes rotating a polishing pad on a first rotation axis; and pressing a substrate rotating on a second rotation axis tilted with respect to the first rotation axis, against the polishing pad to polish the substrate while always keeping the circumferential edge of the substrate inside the circumferential edge of the polishing pad.

FIGS. 1A and 1B are schematic diagrams of the polishing apparatus according to the first embodiment. FIG. 1A is a side view of the polishing apparatus and FIG. 1B is a top view of the polishing apparatus. The polishing apparatus of the first embodiment is a CMP apparatus 100 configured to polish the face of a substrate such as a semiconductor wafer.

The CMP apparatus 100 of the first embodiment includes a turntable 10 (the polishing table), a polishing pad 12, a first supporting shaft 14; a first rotating mechanism 18, a top ring 20 (the substrate holding unit), a second supporting shaft 22, a second rotating mechanism 24, a top ring tilting mechanism 26 (the tilting mechanism), and a polishing agent supplying nozzle 28.

The turntable 10 rotates on a first rotation axis RA1. The turntable 10 rotates on its own axis that is the first rotation axis RA1. The turntable 10 is an example of the polishing table.

The polishing pad 12 is provided on the turntable 10. The polishing pad 12 is fixed on the turntable 10. As in the turntable 10, the polishing pad 12 rotates on the first rotation axis RA1. The polishing pad 12 rotates in the circumferential direction.

The polishing pad 12 is an elastic body. The polishing pad 12 contains resin or nonwoven fabric, for example. The polishing pad 12 is made of polyurethane resin, for example.

The first supporting shaft 14 supports the turntable 10 at the center of the turntable 10.

The first rotating mechanism 18 is designed to rotate the turntable 10. The first rotating mechanism 18 rotates the first supporting shaft 14. The first rotating mechanism 18 includes a motor, and a bearing configured to rotatably hold the first supporting shaft 14, for example.

The top ring 20 is designed to hold a semiconductor wafer W to be polished. The top ring 20 is designed to press the semiconductor wafer W being held against the polishing pad 12.

The top ring 20 rotates on a second rotation axis RA2. The top ring 20 rotates on the second rotation axis RA2. As in the top ring 20, the semiconductor wafer W held by the top ring 20 rotates on the second rotation axis RA2. The top ring 20 is an example of the substrate holding unit.

The second supporting shaft 22 supports the top ring 20 at the center of the top ring 20.

The second rotating mechanism 24 is designed to rotate the top ring 20. The second rotating mechanism 24 rotates the second supporting shaft 22. The second rotating mechanism 24 includes a motor, and a bearing configured to rotatably hold the second supporting shaft 22, for example.

The first rotating mechanism 18 and the second rotating mechanism 24 rotate the turntable 10 and the top ring 20, respectively, in one direction. In FIGS. 1A and 1B, for example, both the turntable 10 and the top ring 20 rotate in the counterclockwise direction.

The top ring tilting mechanism 26 is designed to change the angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 1A) for polishing of the semiconductor wafer W. The top ring tilting mechanism 26 supports the second rotating mechanism 24, for example. The top ring tilting mechanism 26 is designed to fix the angle θ between the first rotation axis RA1 and the second rotation axis RA2 at a constant angle during polishing of the semiconductor wafer W.

The top ring tilting mechanism 26 moves the top ring 20, the second supporting shaft 22, and the second rotating mechanism 24 using a motor, for example, and tilts these components with respect to the vertical direction.

The top ring tilting mechanism 26 is designed to change the angle θ between the first rotation axis RA1 and the second rotation axis RA2 within the range of 0° to 5°, for example.

The top ring tilting mechanism 26 is designed to tilt the second supporting shaft 22 by an angle θ with respect to the vertical direction, for example. Tilting the second supporting shaft 22 changes the inclination of the second rotation axis RA2.

Tilting the second supporting shaft 22 tilts the face of the semiconductor wafer W being polished by the angle θ with respect to the face of the turntable 10. This makes the face of the semiconductor wafer W being polished not in parallel with the face of the turntable 10.

The polishing agent supplying nozzle 28 is designed to supply slurry onto the face of the polishing pad 12. The slurry contains abrasive grains. The abrasive grains are particles containing silicon oxide, aluminum oxide, or cerium oxide, for example.

The CMP apparatus 100 is configured to polish the semiconductor wafer W while always keeping the circumferential edge of the semiconductor wafer W (shown by We in FIG. 1B) inside the circumferential edge of the polishing pad 12 (shown by 12 e in FIG. 1B).

In the CMP apparatus 100, the first rotation axis RA1 of the turntable 10 and the second rotation axis RA2 of the top ring 20 do not move relative to each other during polishing of the semiconductor wafer W. In the CMP apparatus 100, the relative position of the turntable 10 and the top ring 20 does not change during polishing of the semiconductor wafer W. Also, the relative position of the first supporting shaft 14 and the second supporting shaft 22 does not change during polishing of the semiconductor wafer W.

The CMP method according to the first embodiment will now be described. A case where the CMP apparatus 100 of the first embodiment is used will be described as an example.

In the CMP method of the first embodiment, the face of the semiconductor wafer W is polished and planarized. The semiconductor wafer W to be polished has a circuit pattern formed by repeated layer deposition and etching of the film, for example. The semiconductor wafer W to be polished has at least one of a dielectric film and a conductive film exposed on its face.

First, the semiconductor wafer W is put into the CMP apparatus 100.

The turntable 10 is then rotated on the first rotation axis RA1 by the first rotating mechanism 18. The polishing pad 12, which is fixed on the turntable 10, is also rotated on the first rotation axis RA1.

The polishing agent supplying nozzle 28 is then operated to supply slurry onto the face of the polishing pad 12.

The face of the semiconductor wafer W held by the top ring 20 is then pressed against the polishing pad 12. The top ring 20 is rotated on the second rotation axis RA2 by the second rotating mechanism 24. The semiconductor wafer W, which is held by the top ring 20, is also rotated on the second rotation axis RA2.

The top ring tilting mechanism 26 tilts the second supporting shaft 22 by an angle θ with respect to the vertical direction. The second rotation axis RA2 is tilted by the angle θ with respect to the first rotation axis RA1. The second rotation axis RA2 is tilted by the angle θ with respect to the vertical direction.

Tilting the second supporting shaft 22 tilts the face of the semiconductor wafer W by the angle θ with respect to the face of the turntable 10. This makes the face of the semiconductor wafer W not in parallel with the face of the turntable 10.

While the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10, the semiconductor wafer W is pressed against the polishing pad 12. The face of the semiconductor wafer W is thus polished while the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10.

The angle θ between the first rotation axis RA1 and the second rotation axis RA2 is more than 0° and equal to or less than 5°, for example. The angle θ between the first rotation axis RA1 and the second rotation axis RA2 is fixed at a constant angle during polishing.

After the face of the semiconductor wafer W has been polished, the supply of slurry onto the polishing pad 12 is stopped. The semiconductor wafer W is then taken out of the CMP apparatus 100.

The functions and effects of the CMP apparatus 100 and the CMP method of the first embodiment will now be described.

In CMP, a polishing agent is supplied onto a rotating turntable, and then a semiconductor wafer being rotated is pressed against a polishing pad provided on the turntable in order to polish the face of the semiconductor wafer. The face of the semiconductor wafer is thus chemically and mechanically polished and planarized.

The rotation axis of the turntable and the rotation axis of the semiconductor wafer W are generally kept in parallel with each other so that the face of the semiconductor wafer W is uniformly polished. In other words, the face of the semiconductor wafer W and the face of the turntable 10 are kept in parallel with each other during polishing.

However, there is a case where the polishing rate differs between the central area and the circumferential area of the semiconductor wafer W and the face of the semiconductor wafer W may be polished nonuniformly. For example, there is a case where the polishing rate in the central area of the semiconductor wafer W is higher than the polishing rate in the circumferential area of the semiconductor wafer W. The difference in the polishing rate is thought to be caused depending on the type of the film and the pattern formed on the face of the semiconductor wafer W.

The CMP apparatus 100 of the first embodiment includes the top ring tilting mechanism 26. The top ring tilting mechanism 26 is designed to change the angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 1A) for polishing of the semiconductor wafer W.

The top ring tilting mechanism 26 keeps the face of the semiconductor wafer W not in parallel with the face of the turntable 10 during polishing of the face of the semiconductor wafer W. Keeping the face of the semiconductor wafer W not in parallel with the face of the turntable 10 causes different forces for pressing the semiconductor wafer W against the polishing pad 12 across the face of the semiconductor wafer W. The force for pressing the semiconductor wafer W against the polishing pad 12 is greater in the circumferential area of the semiconductor wafer W than in the central area of the semiconductor wafer W. Accordingly, the polishing rate in the circumferential area of the semiconductor wafer W becomes higher than the polishing rate in the central area of the semiconductor wafer W.

The top ring tilting mechanism 26 enables adjustment of the polishing rate across the face of the semiconductor wafer W.

For example, the face of the semiconductor wafer W can be polished uniformly by fixing the angle θ between the first rotation axis RA1 and the second rotation axis RA2 at an appropriate angle depending on the type of the film and the pattern formed on the face of the semiconductor wafer W. For example, fixing the angle θ at a large angle will make the polishing rate in the circumferential area of the semiconductor wafer W higher than the polishing rate in the central area of the semiconductor wafer W.

The top ring tilting mechanism 26 should preferably change the angle θ between the first rotation axis RA1 and the second rotation axis RA2 within the range of 0° to 5°. There is a case where uniform polishing is achieved at an angle θ of 0°. An angle θ more than 50 makes uniform polishing difficult irrespective of the type of the film and the pattern. During polishing of the semiconductor wafer W, the angle θ between the first rotation axis RA1 and the second rotation axis RA2 should preferably be more than 0° and equal to or less than 5°, more preferably equal to or more than 0.5° and equal to or less than 5°, and much more preferably equal to or more than 1° and equal to or less than 4°, for example.

The face of the semiconductor wafer W can be polished uniformly by always keeping the circumferential edge of the semiconductor wafer W inside the circumferential edge of the polishing pad 12 during polishing of the semiconductor wafer W.

In view of uniform polishing of the face of the semiconductor wafer W, the relative position of the first rotation axis RA1 of the turntable 10 and the second rotation axis RA2 of the top ring 20 should preferably be fixed. In other words, the relative position of the first supporting shaft 14 and the second supporting shaft 22 should preferably be fixed during polishing of the semiconductor wafer W.

In view of uniform polishing of the face of the semiconductor wafer W, the turntable 10 and the top ring 20 should preferably be rotated in one direction.

As described above, the first embodiment provides the polishing apparatus and the polishing method that can adjust the polishing rate across the face of the semiconductor wafer W by keeping the face of the semiconductor wafer W not in parallel with the face of the turntable 10. The apparatus and the method thus enable uniform polishing of the face of the semiconductor wafer W.

Second Embodiment

The polishing apparatus of the second embodiment differs from that of the first embodiment in that the tilting mechanism changes the inclination of the first rotation axis instead of the inclination of the second rotation axis. In the following, the description overlapping with the description of the polishing apparatus and the polishing method of the first embodiment will partly be omitted.

FIG. 2 is a schematic diagram of a polishing apparatus according to the second embodiment. FIG. 2 is a side view of the polishing apparatus. The polishing apparatus of the second embodiment is a CMP apparatus 200 configured to polish the face of a substrate such as a semiconductor wafer.

The CMP apparatus 200 of the second embodiment includes the turntable 10 (the polishing table), the polishing pad 12, the first supporting shaft 14, the first rotating mechanism 18, the top ring 20 (the substrate holding unit), the second supporting shaft 22, the second rotating mechanism 24, a turntable tilting mechanism 27 (the tilting mechanism), and the polishing agent supplying nozzle 28.

The turntable tilting mechanism 27 is designed to change the angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 2) for polishing the semiconductor wafer W. The turntable tilting mechanism 27 supports the first rotating mechanism 18, for example. The turntable tilting mechanism 27 is designed to fix the angle θ between the first rotation axis RA1 and the second rotation axis RA2 at a constant angle during polishing of the semiconductor wafer W.

The turntable tilting mechanism 27 tilts the turntable 10, the first supporting shaft 14, and the first rotating mechanism 18 using an oil pressure mechanism, for example, with respect to the vertical direction.

The turntable tilting mechanism 27 is designed to change the angle θ between the first rotation axis RA1 and the second rotation axis RA2 within the range of 0° to 5°, for example.

The turntable tilting mechanism 27 is designed to tilt the first supporting shaft 14 by an angle θ with respect to the vertical direction, for example. Tilting the first supporting shaft 14 changes the inclination of the first rotation axis RA1.

Tilting the first supporting shaft 14 tilts the face of the turntable 10 by the angle θ with respect to the face of the semiconductor wafer W being polished. This makes the face of the semiconductor wafer W being polished not in parallel with the face of the turntable 10.

A CMP method according to the second embodiment will now be described. A case where the CMP apparatus 200 of the second embodiment is used will be described as an example.

First, the semiconductor wafer W is put into the CMP apparatus 200.

The turntable tilting mechanism 27 tilts the first supporting shaft 14 by an angle θ with respect to the vertical direction. The first rotation axis RA1 is tilted by the angle θ with respect to the second rotation axis RA2. The first rotation axis RA1 is tilted by the angle θ with respect to the vertical direction.

The turntable 10 is then rotated on the first rotation axis RA1 by the first rotating mechanism 18. The polishing pad 12, which is fixed on the turntable 10, is also rotated on the first rotation axis RA1.

The polishing agent supplying nozzle 28 is then operated to supply slurry onto the face of the polishing pad 12.

The face of the semiconductor wafer W held by the top ring 20 is then pressed against the polishing pad 12. The top ring 20 is rotated on the second rotation axis RA2 by the second rotating mechanism 24. The semiconductor wafer W, which is held by the top ring 20, is also rotated on the second rotation axis RA2.

While the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10, the semiconductor wafer W is pressed against the polishing pad 12. The face of the semiconductor wafer W is thus polished while the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10.

The angle θ between the first rotation axis RA1 and the second rotation axis RA2 is more than 0° and equal to or less than 5°, for example, and fixed at a constant angle during polishing.

After the face of the semiconductor wafer W has been polished, the supply of slurry onto the polishing pad 12 is stopped. The semiconductor wafer W is then taken out of the CMP apparatus 200.

As described above, the second embodiment provides the polishing apparatus and the polishing method that can adjust the polishing rate across the face of the semiconductor wafer W by keeping the face of the semiconductor wafer W not in parallel with the face of the turntable 10, as in the first embodiment. The apparatus and the method thus enable uniform polishing of the face of the semiconductor wafer W.

Third Embodiment

The polishing apparatus of the third embodiment differs from that of the first embodiment in that the apparatus further includes a control unit configured to control the tilting mechanism and change the angle between the first rotation axis and the second rotation axis during polishing of the substrate. In the following, the description overlapping with the description of the polishing apparatus and the polishing method of the first embodiment will partly be omitted.

FIG. 3 is a schematic diagram of a polishing apparatus according to the third embodiment. FIG. 3 is a side view of the polishing apparatus. The polishing apparatus of the third embodiment is a CMP apparatus 300 configured to polish the face of a substrate such as a semiconductor wafer.

The CMP apparatus 300 of the third embodiment includes: the turntable 10 (the polishing table); the polishing pad 12; the first supporting shaft 14; the first rotating mechanism 18; the top ring 20 (the substrate holding unit); the second supporting shaft 22; the second rotating mechanism 24; the top ring tilting mechanism 26 (the tilting mechanism); the polishing agent supplying nozzle 28; and a tilting mechanism controlling unit 30 (the control unit).

The tilting mechanism controlling unit 30 controls the top ring tilting mechanism 26. The tilting mechanism controlling unit 30 is designed to control the top ring tilting mechanism 26 and change the angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 3) during polishing of the semiconductor wafer W.

The tilting mechanism controlling unit 30 is hardware such as a circuit board, or a combination of such hardware and software such as a control program stored in the hardware and a memory, for example.

A CMP method according to the third embodiment will now be described. A case where the CMP apparatus 300 of the third embodiment is used will be described as an example.

First, the semiconductor wafer W is put into the CMP apparatus 300.

The turntable 10 is then rotated on the first rotation axis RA1 by the first rotating mechanism 18. The polishing pad 12, which is fixed on the turntable 10, is also rotated on the first rotation axis RA1.

The polishing agent supplying nozzle 28 is then operated to supply slurry onto the face of the polishing pad 12.

The face of the semiconductor wafer W held by the top ring 20 is then pressed against the polishing pad 12. The top ring 20 is rotated on the second rotation axis RA2 by the second rotating mechanism 24. The semiconductor wafer W, which is held by the top ring 20, is also rotated on the second rotation axis RA2.

The top ring tilting mechanism 26 keeps the second supporting shaft 22 in parallel with the vertical direction. In other words, the top ring tilting mechanism 26 keeps the second rotation axis RA2 in parallel with the first rotation axis RA1. The angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 3) is 0°. The face of the semiconductor wafer W and the face of the turntable 10 are kept in parallel with each other during polishing.

After the face of the semiconductor wafer W has been polished for a predetermined period of time, the tilting mechanism controlling unit 30 controls the top ring tilting mechanism 26 to tilt the second supporting shaft 22 with respect to the vertical direction. The second rotation axis RA2 is tilted by an angle θ (θ>0°) with respect to the first rotation axis RA1. The second rotation axis RA2 is tilted by the angle θ with respect to the vertical direction.

The second supporting shaft 22 is tilted while the turntable 10 is being rotated.

Tilting the second supporting shaft 22 tilts the face of the semiconductor wafer W by the angle θ with respect to the face of the turntable 10. This makes the face of the semiconductor wafer W not in parallel with the face of the turntable 10.

While the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10, the semiconductor wafer W is pressed against the polishing pad 12. The face of the semiconductor wafer W is polished for a predetermined period of time while the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10.

After the face of the semiconductor wafer W has been polished, the supply of slurry onto the polishing pad 12 is stopped. The semiconductor wafer W is then taken out of the CMP apparatus 300.

According to the CMP method of the third embodiment, a first polishing step in which the face of the semiconductor wafer W is kept in parallel with the face of the turntable 10 is followed, without a break, by a second polishing step in which the face of the semiconductor wafer W is kept not in parallel with the face of the turntable 10. The polishing rate in the circumferential area of the semiconductor wafer W is higher in the second polishing step than in the first polishing step.

According to the CMP apparatus 300 and the CMP method of the third embodiment, the polishing rate across the face of the semiconductor wafer W can minutely be adjusted by changing the angle between the first rotation axis RA1 and the second rotation axis RA2 (the angle θ in FIG. 3) during polishing. The apparatus and the method thus further facilitate uniform polishing of the face of the semiconductor wafer W. In addition, since the angle θ between the first rotation axis RA1 and the second rotation axis RA2 is changed without a break during polishing, the throughput in the polishing process is improved compared to the case with a break between two polishing steps, for example.

In the above CMP method, the angle θ between the first rotation axis RA1 and the second rotation axis RA2 is changed from an angle θ of 0θ in the first polishing step to an angle θ more than 0° in the second polishing step. The CMP method, however, is not limited to this example. For example, the method may include a first polishing step with an angle θ more than 0° and a second polishing step with an angle θ of 0°. Alternatively, the method may include a first polishing step with an angle θ more than 0° and a second polishing step with an angle θ more than the angle θ of the first polishing step. Alternatively, the method may include a third polishing step with an angle θ different from those of first and second polishing steps. Alternatively, the method may include a first polishing step with an angle θ, a second polishing step with an angle θ more than the angle θ of the first polishing step, and a third polishing step with an angle θ less than the angle θ of the second polishing step.

As described above, the third embodiment provides the polishing apparatus and the polishing method that can adjust the polishing rate across the face of the semiconductor wafer W by keeping the face of the semiconductor wafer W not in parallel with the face of the turntable 10, as in the first embodiment. The apparatus and the method thus enable uniform polishing of the face of the semiconductor wafer W. In addition, the apparatus with the control unit enables minute adjustment of the polishing rate across the face of the semiconductor wafer W. The apparatus and the method thus further facilitate uniform polishing of the face of the semiconductor wafer W.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the polishing apparatuses and the polishing methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the apparatuses and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A polishing apparatus comprising: a polishing table; a polishing pad provided on the polishing table; a first rotating mechanism configured to rotate the polishing table on a first rotation axis; a substrate holding unit configured to hold a substrate and press the substrate against the polishing pad; a second rotating mechanism configured to rotate the substrate holding unit on a second rotation axis; and a tilting mechanism configured to change an angle between the first rotation axis and the second rotation axis, wherein a circumferential edge of the substrate is always kept inside a circumferential edge of the polishing pad during polishing of the substrate.
 2. The polishing apparatus according to claim 1, wherein a relative position of the first rotation axis and the second rotation axis is fixed during polishing of the substrate.
 3. The polishing apparatus according to claim 1, wherein the tilting mechanism changes the angle within a range of 0° to 5°.
 4. The polishing apparatus according to claim 1, wherein the tilting mechanism changes an inclination of the second rotation axis.
 5. The polishing apparatus according to claim 1, further comprising: a control unit configured to control the tilting mechanism and change the angle during polishing of the substrate.
 6. The polishing apparatus according to claim 1, wherein the first rotating mechanism and the second rotating mechanism rotate the polishing table and the substrate holding unit, respectively, in one direction.
 7. A polishing method comprising: rotating a polishing pad on a first rotation axis; and pressing a substrate rotating on a second rotation axis tilted with respect to the first rotation axis, against the polishing pad to polish the substrate while always keeping a circumferential edge of the substrate inside a circumferential edge of the polishing pad.
 8. The polishing method according to claim 7, wherein an angle between the first rotation axis and the second rotation axis is more than 0° and equal to or less than 5°.
 9. The polishing method according to claim 7, wherein the angle between the first rotation axis and the second rotation axis is changed during polishing of the substrate.
 10. The polishing method according to claim 7, wherein the polishing table and the substrate holding unit are rotated in one direction. 